Process and device yield in optical lithography imaging processes are directly related to Critical Dimension (CD) uniformity. CD uniformity is dependent on several processes during the optical lithography process, such as imaging, etching, and deposition. In the lithography process, there are several factors that influence the CD uniformity on a wafer, such as reticle uniformity, slit uniformity, wafer flatness, lens aberrations, and imaging focus. Typically, these factors are tested individually using a variety of tests that may be time consuming, require specialized hardware to perform, and/or require technicians who have received specialized training to perform the tests.
Typical methods for determining parameters of an exposure tool, such as scan direction effects, field attributes, and lens system aberrations cannot be performed without severely disrupting the normal manufacturing process on the exposure tool. In addition, the typical methods fail to efficiently and effectively organize and analyze the large amounts of data needed to accurately and precisely determine the parameters.
Typically, projection lenses for exposure tools in the semiconductor industry have adjustable lens elements for correcting for lens aberrations. Correcting for lens aberrations in some tools may be performed by adjusting the position and tilt of elements within the lens system. Tool vendors typically adjust the lens elements during the calibration of the exposure tools. The majority of calibration procedures require a specially trained service or maintenance engineer and specialized hardware to perform. In addition, the calibration procedures are usually time consuming requiring significant downtime on the exposure tool.
A typical lens system includes many lens elements. Aberrations in a lens system can change over time due to the aging of the lens system materials, environmental effects, or the non-linearity of control algorithms used to adjust the lens system. For example, each lens has a heating curve associated with it, such that as the lens heats up due to environmental conditions or due to lens use during exposures, the effective focus length of the lens changes. Air pressure also has a predictable effect on the lens elements and their focus values. Aberrations in the lens system can also change due to maintenance events or other mechanical effects, such as shipping. Control algorithms in the exposure tools are typically used to adjust one or more of the lens elements to compensate for measured external effects or internal effects.
CD control and image integrity of device layers is a direct function of several components, including dose and focus of the exposure tool. Typically, dose feedback is an active run to run control parameter. Focus feedback, however, typically has not been an active run to run control parameter. Typically, the optimal focus setting for any given product/tool/layer/reticle context value combination is determined at the context inception and used throughout the life of the product. In the event that an intrusive tool event occurs and the tool baseline focus is lost or changed, the process set point for each context value is reestablished. Typical ex-situ tool focus monitoring techniques have not exhibited the accuracy and precision to substantiate product process set point changes based on measured focus values. These techniques have typically been used only for monitoring by providing flags for obvious large focus excursions.
Focus is typically controlled through explicit context value control. The best focus process point is typically determined by evaluating focus exposure process windows at the time of the new context introduction. This best focus process value is then used for the lifetime of the context value. A disadvantage of this process is that there is no process available to reset the focus values in the presence of tool baseline focus shifts or to correct for uncompensated focus drifts in the exposure tool. In the event of a large change in the tool focus, there is no direct method to apply the new setting to the context data.
Exposure tool focus offsets induced on product as a result of in-situ focus sensor systems inability to measure edge of substrate image fields and large focus rate of change of topographical features can result in significant process and device yield loss due to poor focus plane determination and fitting. Typically, exposure tools have significant problems determining focal image planes on edge die or over sever topography. Typical exposure tools require some fitting functions from neighboring fields or a partial system shutdown to prevent erroneous data from being used in the fitting functions.
Dark field microscopy and inspection are fundamental arts of inspection in many industries. There are several components of the inspection tool hardware that contribute to the illumination of the sample in darkfield inspection, such as the illumination source itself, the beam delivery hardware, the darkfield splitter hardware, the lens objective design, and the camera adapter. Each of these components plays a significant role in the illumination of the sample and the collection of the darkfield image formed from the sample. Typical methods provide for illumination uniformity measurements along the Cartesian x and y axis. This is insufficient. Illumination uniformity measurements along the Cartesian x and y axis do not allow the investigation of the entire circumference of the system pupils in azimuthal increments.